Impact welding - surface treatments lower velocity and the energy threshold to weld:

Impact welding has the considerable benefit of joining dissimilar materials that cannot be joined by other means. Of particular interest are the joining of aluminium alloys to steel for use in aerospace and automotive structures and the joining of stainless and exotic materials and alloys to steel such as titanium, zirconium, tantalum, copper, hastelloy, monel, and many other nickel and chromium alloys. 

There are two principle methods of propelling one part into another to form a weld - first is by fast burning explosives and second by electro-magnetic induction/repulsion. Explosives are used to form welds over large relatively flat surfaces on the one hand and for expanding pipes like flue pipes in boiler end plates on the other. Magnetic welds can only be formed with projectile metals that are good conductors, or that can have a good conductor wrapped around it (referred to as a flyer ring) because the process relies upon inducing a current in the moving part that creates a magnetic field that opposes the magnetic field that induces the current, thus parts tend to be circular or co-axial.

Magnetic pulse welding is used on smaller parts and the actual magnetic welds can take the form of a narrow circumferential band of molecular bonded material like a cold pressure weld or alternatively a rather larger area of distributed asperity cold pressure welds interspersed with un-welded areas, these welded asperity fields held together by an outer constraint, and this latter joint behave more like a swaged mechanical friction joint. The concept of using distributed asperity welds is akin to Trib-Joints, which are high friction joints made by pushing one part into another as in a press fit.

The practical usefulness of impact or projectile welding of metals is seriously limited by practical difficulties associated with applying an intense pulse of energy, sufficient to accelerate and impact one body against another at velocities of about 300m/s colliding at an oblique angle averaging 8° without damaging the parts being joined! 

Trib-Tools are used to apply a chemical agent that conditions (prepares) the surfaces to be welded in specific ways. The effect of the treatment is to lower the threshold velocity at which welding occurs from typically 300m/s (meters/second) to below 150m/s and this therefore lowers the energy to weld. The theoretical relationship between velocity and energy is an inverse square law, however the impact welding process relies upon a self cleaning scouring process during impact, the effectiveness of which is dependant upon the velocity of impact, thus a process that simply reduces the velocity threshold to weld may cause a deterioration in weld quality unless steps are taken to compensate for the reduced scouring efficiency. 

The scouring jet is a semi molten jet of material sheared from the contacting surfaces and ejected from the contact front as it moves forward, the jet scouring the metals ahead of the contact clean of oxides therefore the clean faces naturally weld upon forceful contact. As the velocity of impact is reduced so the velocity and energy of the scouring jet is lowered, thus for reliable welding at lower velocity the surface roughness must also be optimised during surface preparation with a Trib-Tool to reduce the work to be done by the scouring jet. The net result is that energy savings are somewhat less than might be expected from the theoretical inverse square law. Practical tests have shown the actual energy reduction can be of the order of 50% rather than the 90% theoretical. This is still a very useful improvement and significantly reduces the practical difficulties of applying both explosive and magnetic pulse welding processes.

Patents are pending on both the use of a chemical agent to treat the surface and the  mechanical means of treatment. When used together these result in a significant reduction of the impact velocity threshold and an improvement in weld quality. 

To view a micro-graph of an impact weld joint made with the help of Trib-Tools click on explosive welding below.

There are two propulsion systems in common use - explosive, which are fast burning charges and electromagnetic induction, click on the following for further information:

for more information email: info8@tribtech.com

The TribTech name derives from "tribos" - Greek for 'rubbing'. 'TribTech' is a trade name used by Ball Burnishing Machine Tools Ltd. of 12 Brookmans Av. Hatfield, Herts. AL9 7QJ. United Kingdom;  Company Reg. No. 1408807, VAT Reg. No. 421 6210 04; a knowledge based company that develops, patents and licenses technology. All rights reserved by Ball Burnishing Machine Tools Ltd. Last modified: May 01, 2008 copyright © 1999/2008. The information and data provided herein should be considered generally representative for the tools and technologies described. In all cases users should carefully evaluate the tools and technologies to determine their suitability for a particular purpose.